1. Field of the Disclosure
This disclosure relates to an apparatus and method for filtering liquids containing suspended solids. Specifically, this disclosure relates to an improved vertical filtration unit.
2. Description of Related Art
The removal of suspended solids from liquids, especially water, has been a longstanding requirement for many uses of such liquids. For example, water, which is free of suspended solids, is greatly preferred for uses, such as drinking, industrial processing, and swimming pools. Vertical filters, which utilize filter media, such as sand and garnet, have been used to separate suspended solids from water.
Vertical filters operate by introducing the liquid containing suspended solids onto the top portion of a bed of filter media. As the liquid passes through the filter media under the force of gravity, the suspended solids adhere to the filter media and/or to solids accumulated on the filer media, allowing liquid essentially free of suspended solids to flow through one or more screen cartridges prior to accumulating in an effluent chamber and then exit from the filtration unit.
The above-described method, although effective at removing suspended solids from liquids, has several shortcomings. The filter media eventually becomes saturated with suspended solids and no longer effectively removes suspended solids from the incoming liquid. When this condition occurs, the vertical filtration unit must cease operation so that the captured suspended solids can be removed from the filter media. This removal is accomplished through an operation referred to as “backwashing,” in which clean liquid is introduced, under pressure, to the bottom of the filter media bed and passes through the filter media in the reverse direction from the filtration process. The reverse flow of the liquid removes the filtered solids from the filter media and exits the vertical filter. This operation is typically continued until the exiting backwash water is essentially free of suspended solids, indicating that the filter media is clean. Backwashing can be very time consuming and generates large volumes of wastewater. Additionally, filtered solids are never completely removed from the filter media during backwashing, which eventually results in the filter media needing to be replaced. Filtration devices utilizing the above method are disclosed in U.S. Pat. No. 650,611.
The problem of backwashing filter media has been addressed, to some extent, through the development of continuous or moving bed filtration systems. While these continuous filtration systems operate to remove suspended solids as outlined above, they have the added benefit of continuously removing filtered suspended solids from the filter media. Examples of continuous filtration systems and various improvements thereon are disclosed and explained in U.S. Pat. Nos. 4,060,484; 4,891,142; 5,582,722; 5,730,886; 6,319,413; 6,517,712; 6,641,737; 6,790,351 all of which are herein incorporated by reference. See also the following articles: “Deep Moving Bed Downflow Sand Filter—An Optimum Granular Media Filter for Wastewater Treatment” American Filtration Society, Volume 10, 1996; and “The Application of Dual Media Concept to Upflow Filtration”, Water Treatment, Volume 4, No. 2, April 1989.
The above-described continuous systems for the filtration of liquids and the removal of suspended solid granules are typically not as effective or efficient as industry desires. One problem that remains is the corrosion of metallic screens in the screen cartridges, for example when brines are processed in a vertical granular media filter such as described above. While various plastic screens have tried, they have met with limited success, primarily due to insufficient strength of plastic screens, and the added cost of engineering special screens for specific applications. Therefore, there remains an unmet need to provide reliable, maintenance-free, continuous, self-cleaning, vertical filtration units for the removal of suspended solids from liquids that may be corrosive to metallic components, such as metallic screens in screen cartridges.